The first example of prior arts relates to a technology of detecting a location of a sound source (hereinafter, referred to also as a “source location”). In the first example of prior arts, it has proposed that sound signals which a plurality of microphones receive are delayed one by one to be convoluted, then a two power of an error among the delayed sound signals is calculated, and eventually a direction of a sound source (hereinafter, referred to also as a “source direction”) of the sound signals is determined from a delay amount having a minimum value of the two power of the error (refer to Patent Reference 1, for example).
The second example of prior arts relates to a technology of detecting a source location of an extraction sound which is extracted from a mixed sound including a noise. In the second example of prior arts, it has proposed that the number of engine rotations of an engine sound of a user's vehicle is detected, then a fundamental wave component and harmonic wave components related to the number of engine rations are removed, and thereby the influence of the engine sound produced by the user's vehicle is reduced (refer to Patent Reference 2, for example).
FIG. 1 shows graphs for explaining a method of removing an engine sound of a user's vehicle according to the second example of prior arts.
In the second example of prior arts, firstly, a fundamental wave component fkHz of the engine sound is determined using a sensor which detects the number of engine rotations of the user's vehicle. Next, using the fundamental wave component fkHz, harmonic wave components 2fkHz, 3fkHz, . . . of the engine sound are determined. FIG. 1 (a) shows frequency signals having a waveform of a sound collected by an acoustic microphone. Then, a horizontal axis represents a frequency, and a vertical axis represents a signal level. Here, signals in frequency bands of the previously determined fundamental wave component fkHz and harmonic wave components 2fkHz, 3fkHz, . . . (refer to FIG. 1 (b)) are removed from the frequency signals of FIG. 1 (a). As a result, as shown in FIG. 1 (c), it is possible to obtain frequency signals from which the signals in frequency bands of the fundamental wave component fkHz and the harmonic wave components 2fkHz, 3fkHz, . . . of the engine sound are removed. Using the frequency signals obtained after the removing, a source location of a vehicle near the user's vehicle is determined.    [Patent Reference 1] Japanese Unexamined Patent Application Publication No. 6-83371 (Claim 2)    [Patent Reference 2] Japanese Unexamined Patent Application Publication No. 5-85288 (Claim 2, FIG. 4)